Micromill



D. B. PALL Jan. 4, 1944.

MICRO-MILL Filed Sept. l0, 1941 m M n Q MTP INVENTOR DAVID B. PAI-L mm .A565

ATTORNEY Patented Jan. 4, 1944 UNITED STATESV PATENT OFFICE MICROMTLL David B. Pall, New York, N. Y., assignor to Interchemical Corporation, New York, N. Y., a corporation of Ohio This invention/relates to the grinding of pigments, and it ist particularly concerned with a new type of laboratory micro-mill.

In the laboratory testing of pigments one of the most dicult problems has been to devisev a method of obtainingaliesuniform results and v and there is no satisfactory way to prevent dirt or dust from becoming dispersed along with the pigment. The second method has the disadvantage that relatively larg'e quantities of pigment must be ground (e. g., a hundred grams or more) in order to obtain uniform results approximatin-g plant conditions. Also, excessive labor is required to keep the mill free from contamination,l and especially to clean it before grinding the next sample.

This invention provides a simple, easily constructed laboratory micro-mill which will produce pigment dispersions closely duplicating plant products in flneness and uniformity, and the mill is particularly suitable forl preparing such dispersions from relatively small quantities of pigment (e. g., a few grams), thus allowing the testing of small samples and avoiding a large waste of material. The mill is easily cleaned and the material being ground remains free from contamination during the entire grinding operation.

The micro-mill of this invention is unique in that its grinding surfaces are made entirely of glass (or similar vitreous material), and in the most practical form these glass surfaces are cone shaped, one fitting Within the other. Various mechanical means may be employed to operate the grinding surfaces,A provided the means are such las will permit easy assembly and manipulation of the grinding surfaces and accurate control of the grinding pressures. A preferred means which easily provides the control necessary is included as part of the complete mill herein described.

Although the detailed form of the mill is capable of numerous variations. a particularly satisfactory embodiment of this invention. is illustrated in the accompanying drawing in which Fig. 1 is a. three-dimensional view of the male and female grinding elements of the mill; and

Fig. 2 is a view, partly in cross section, of -a complete motor driven mill utilizing the grinding elements shown in Fig. 1.

Referring to Fig. 1, a male element A having a grinding surface I0, is designed to twithin a female element B Whichhas a grinding surface Il. 'Ihe operation of the mill comprises forcing a mixture of pigment and vehicle into the open end of the female element B, and between the grinding surfaces I0 and Il while one of the elements is being rotated with respect to the other. The grinding surface l0 of the male element A is made of glass and is conical in shape. A tab l2 is attached* to one end of the element A adjacent that part of the conical surface I0 which has the larger diameter. The tab i2 may be any shape which may be easily gripped to provide rotation of the element A. At the other.

or feed, end of the surface I0, the element A;

is preferably tapered, or beveled, although this is not essential to the operation of the mill. The surface i0 is sanded or otherwise roughened to provide the type of matte surface commonly referred to as ground glass (e. g., roughened with 300 mesh carborundum powder). The surface I0 also bears a V-shaped groove I l, .the apex of the V being in a direction opposite to the rotation of the element A. The leg of the groove I4 which is adjacent to the feed end of the element A has a fairly sharp edge on the side away from the direction of rotation and a beveled or smooth edge where it joins the surface I0 on the forward side. Therefore, this leg of the groove aids in moving the material being ground from the feed end of the element A to between the grinding surfaces l0 and Il. of the groove ilares into a tapered cut I6 across the end of the element A. This tapered cut y,

I'S extends to the center of the feed end of the element A and has a shoulder on the same side (away from the direction of rotation) as'has the first leg of the groove I 4. The other side of the tapered cut 'l 6 is beveled smooth so that the tapered cut resembles the beginning of a helical feed system, and thus aids the transfer of material to the V groove. The lother leg of the groove Il has a relatively steep cut or shoulder on the side toward the direction of rotation and is beveled or smoothed into the surface I0 on the side away from they direction of rotation. This leg of the groove I4 tends to keep the mixture of pigment and vehicle from leaving the grinding surfaces too soon. Also, this leg'aids in main-- Also, the feed end taining the grinding surfaces I8 and II properly lubricated with the material being ground.

The grinding surface II of the female element B is also conical and is designed to fit snugly around the grinding surface I of the element A when the two elements are in grinding position. .The surface II is also made of glass and is roughened like the surface I0. Since considerable pressure is developed it is desirable to reinforce the wall of the element B around the surface II. 'This may conveniently be done by designing the entire element of glass, with glass reinforcing rings I3. The remaining part of the element B, designated as part I5, is a cylindrical tube in which the pigment and vehicle are placed prior to grinding.

Referring to Fig. 2, the mill comprises the grinding elements A and B in grinding position, a motor C for rotating the element A, hydraulic means D and E for urging the elements A and B together, and pneumatic means F for control- Aling the pressures exerted by the hydraulic means D and E. The male grinding element A is rotated by means of a slotted fiber cylinder 26 which fits snugly over the tab I2. The cylinder 20 is keyed to a shaft 22 on which is rigidly mounted a gear wheel 24. The gear wheel 24 meshes with a Worm 26 which in turn is driven by the motor C.

The element A is forced into the element B by means of a plunger 28 (having a diameter of 1 inch) to which is attached a tubular member 30. The member 30 is provided with centering screws 32 which fasten into a gear housing 33 and force it against the shoulder of the gear 24, which in turn forces the element A into the element B.-

The plunger 28 is slidably mounted in a cylinder 34. A pipe 36 in the form of a right angle bend is welded to the cylinder 34 and extends vertically therefrom. The pipe 36 and that portion of space in the cylinder Y34 which is not occupied by the plunger 28 are lled with oil 38. The vertical end of the pipe 36 is welded to a plug 40 containing two vents. One of these vents is normally closed in operation by a plug 42 and the other vent is connected to the pneumatic system F via a line 44. The plunger 28 may have annular grooves 46 cut in its surface to reduce friction in the cylinder 34, due to the close t necessary to prevent leakage of oil, and it is provided with a washer 48, preferably made of leather, at the end which is in contact with the oil 38.

The female grinding element B is held rigidly in place by means of a fixed clamp 2| and a rubber block 23. The mixture of vehicle and pigment to be ground is placed in the part I5 of the element B and a plunger 25, preferably made of steel, is inserted in the end of the part I5. In order that the material to be ground does not lead along the plunger'25, a rubber end, or washer, 21 which ts Vsnugly in the part I5 is provided in the end of the plunger 25.V The material to be ground is forced between the grinding surfaces by exerting pressure against the plunger 25 through the medium of the hydraulic means E which is identical in construction and operation with the hydraulic means D-already described. The hydraulic means E is also connected tothe pneumatic system F through a line 23.

After the mixture of pigment and vehicle has been inserted in the end I5, and the plunger'25 and the elements A and B are in position, the motor p is started which causes the rotation of flcient vehicle between the grinding surfaces I0 permit them to slide.

and II to prevent binding or freezing, the male element A is forced into the female element B by bringing pressure to Ibear on the housing 33 by means of the plunger 28. At the same time the material to be ground is forced against the end of the grinding element A 'by pressure exerted against the plunger 25. In order to maintain sufficient pressure between the grinding surfaces II! and II to effect uniform dispersion of the pigment it is essential that the feed force exerted by the plunger 25 at no time exceeds the grinding vforce exerted by the plunger 28, and in ordi-` nary operation the grinding force exerted by the.

plunger 28 is always considerably in excess of the feed force. 'I'he balance between these pressures yis maintained by means of the pneumatic system F. In this system compressed air is admitted through a valve 50 in the line 44 until the air pressure over the oil 38 reaches the desired amount, as is indicated on a gauge 5I. 'I'he air pressure in the line 44 is transmitted to the line 29 via a differential pressure valve 52 which may be set to maintain the same or less air pressure in the line 29, as compared to the line 44. The pressure of the air in the line 29, which is indicated -by a gauge 53, is exerted against the oil in the hydraulic means E and is thereby transmitted to the plunger 25. When the grinding operation is finished the grinding elements A and B may be separated by closing the valve 50 and opening a bypass valve 54 and a vent valve 55 and pulling the grinding element A away from the element B.

In order that the gear 24 will always remain meshed with the worm 26 the motor C is mounted on legs 3| which are rounded at the ends to The 'gear 24 and the worm 26 are heldin mesh by means of the gear housing 33 which moves longitudinally with the shaft 22 and thereby carries the motor C along with it.

Thedispersion of pigment in vehicle is discharged from the mill at the ends of the grinding surfaces I0 and II adjacent the tab I2, and it may be collected in a receptacle 35.

In order to compare the results obtained with this mill with those obtained by standard methods, a typical laboratory produced titanium dioxide pigment was mixed with a rened'linseed oil varnish. One-third of the mixture was mulled on a ground porcelain slab, one-third was mulled on a glass slab and the last third was passed twice through the micro-mill under a pressure difference across the valve 52 of 20 pounds per square inch, each pass requiring four minutes. The dispersion obtained from the porcelain slab has a reflectance (relative to magnesium oxide) equal to 83.30%, the dispersion from the glass slab had a reflectance equal to 90.15% and the material from the micro-mill had a reflectance equal to 94.45%.

In another test, using a somewhat harder, grit- In another test with a hard grinding iron blue pigment in a refined linseed oil varnish the dispersion was developed to within 1% of its ultimate tinting strength in four passes requiring about ten minutes each for a quantity of three grams. The ultimate strength of the dispersion ground on the three roll mill was slightly higher than the material produced by the micro-mill. On the other hand the dispersion was about weaker than the product from the roll mill when it was prepared by 400 strokes with a pound muller. With another iron blue pigment of only moderate hardness the strengths developed by the roll mill and the micro-mill were the same.

In the'case of very thin dispersions, such as are encountered in certain types of inks, the micromill will complete a grind in less time than the three roll mill, and in the case of very heavy inks it is faster than hand mulling.

The force on'the plunger feeding the material will vary, depending on the nature of the material being ground, the force exerted against the male grinding element (in other words, the

grinding pressure) and the results desired. The force with which the male grinding element is urged into the female element depends on the hardness of the pigment being ground, the thickness of the dispersion, the fineness of dispersion desired and the size and shape of the grinding surfaces of the male and female elements. For grinding practically all pigments and vehicles the mill -should be designed to develop a pressure between the grinding surfaces as high as about 250 pounds per square inch, although satisfactory dispersions with most pigment and vehicle combinations will be obtained at lower grinding pressures. 'In .the mill shown in the drawing this high grinding pressure is developed with relatively low forces (or pressures) on the plungers because of the rather slight taper of the conical grinding surfaces. This taper is, course, not critical, but too sharp a taper requires excessive pressures on the plungers and too slight a taper makes control of the grinding pressure difiicult, with consequent frequent freez.. in g and breakage. In the mill shown in the drawing, the male grinding element wasa standard solid glass stopper with the small end beveled off to a depth of about 1A; inch and the V groove cut in the grinding surface. The dimensions were those of the standard 15/35 ground glass joint.

The V groove is not essential, but it is quite difllcult to obtain appreciable output from the mill without it. Preferably, the first leg of the groove (at the feed end) should be at an angle of about 45 to an element of the conical surface,

and the other leg shouldbe at an angle of ap the progress of the dispersion through the mill can be observed at all times. Thus, it is easy to adjust the feed and grinding pressures to give a slow, steady delivery of the dispersion. Also, the

male and female grinding elements are so simple and inexpensive that a number of diiferent sets of them may conveniently be kept in stock for use with particular pigments and vehicles.

The millwill operate eiliciently with dispersions of widely different viscosities, and it will conveniently handle mixtures which are too heavy or too 'drygo/ull by hand. Even with the heaviest materials 'the heat developed by the grindinghas not shown an appreciable effect on the finished dispersions, although al water jacketed female grinding element can be employed where unusually accurate temperature control is desired.

It is essential, in this mill, that the grinding surfaces be made of glass (or other vitreous material) in order to avoid contamination of the material being ground and to provide a reasonably eiiicient grinding surface, and from the practical standpoint the entire grinding elements should be made of glass. The mill itself can, of course, be operated in any position, although the hydraulic means for pressure control described (if it is employed) will obviously have to be altered to prevent spillageof oil if the mill is operated in a position much different from horizontal. Also, the male grinding element maybe held stationary and the female element rotated, or both rotated in opposite directions.

Other variations of construction will be apparent to those skilled in the art. For example, a multiple female grinding element may be made which will accommodate a plurality of male elements, with one feed chamber, and similarly, two male grinding elements may be joined backto-back to accommodate two separate female elements. Also,` it is not necessary that the grinding and feeding ends of the female element be in a straight line, although this is the simplest construction. Similarly, the male element may bear a series of V grooves, or the first leg of the V may be omitted entirely. The second leg, however, is necessary to obtain easy grinding and uniform results. The angles at which the legs of the groove are cut are not critical. Obviously, if the second leg of the groove is cut along an element of the conicalsurface, the material will l essential, but it aids greatly in transferring the pigment and vehicle to the grinding surfaces. Alternatively, the female grinding element may bear all or part of the V groove instead of the maleA element.

'I'he V groove and tapered cut are desirable only Where the mill is operated under a xed grinding pressure. That is, if the mill is designed to maintain a xed clearance between the grinding surfaces, the groove is not needed, but it is not practical, from the standpoint of construction, to design a mill of this type with fixed clearance instead of control by grinding pressures.

I claim:

1. A laboratory micro-mill suitable for dispersion of pigments in vehicles, which comprises glass male and female grinding elements the grinding surfaces of which are corneal in shape and of a, du1l or matte appearance, means for rotating one grinding element relative to the other element, hydraulic means for urging said male element into said female element, hydraulic means for feeding material against the end of said male element and between said grinding surfaces, and pneumatic meansfor operating and controlling said hydraulic means so that the feed pressure exerted on the material to be ground does not exceed'the pressure urging said male element into said female element.

2. In a laboratory type micro mill, an all glass male grinding element having feed and discharge ends and a conical grinding surface of dull or matte appearance, with a groove in said surface comprising twq connecting legs lying on thef same side of an element of said conical surface, the' leg adjacent the feed end making an angle of about 45 F. with the lelement of said' surface, and the leg adjacent the discharge end making an angle of about 15 with the element of said surface, and a. tapered cut in the feed end of said male element cooperating with the feed end leg of said groove,

3. In a laboratory type micro mill, an all glass essaies a substantially V-shaped groove is obtained, and

a tapered cut in the feed end of said/maleelement cooperating with the feed end leg of said DAVID B. PALL. 

